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123 lines
4.1 KiB
Markdown
123 lines
4.1 KiB
Markdown
# Interlaced Video
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Video buffers have a number of states identifiable through a combination
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of caps and buffer flags.
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Possible states:
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- Progressive
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- Interlaced
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- Plain
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- One field
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- Two fields
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- Three fields - this should be a progressive buffer with a repeated 'first'
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field that can be used for telecine pulldown
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- Telecine
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- One field
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- Two fields
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- Progressive
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- Interlaced (a.k.a. 'mixed'; the fields are from different frames)
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- Three fields - this should be a progressive buffer with a repeated 'first'
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field that can be used for telecine pulldown
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Note: It can be seen that the difference between the plain interlaced
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and telecine states is that in the latter, buffers containing
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two fields may be progressive.
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Tools for identification:
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- `GstVideoInfo`
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- `GstVideoInterlaceMode` - enum `GST_VIDEO_INTERLACE_MODE_...`
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- PROGRESSIVE
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- INTERLEAVED
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- MIXED
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- Buffers flags - `GST_VIDEO_BUFFER_FLAG_...`
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- TFF
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- RFF
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- ONEFIELD
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- INTERLACED
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## Identification of Buffer States
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Note that flags are not necessarily interpreted in the same way for all
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different states nor are they necessarily required nor make sense in all
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cases.
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### Progressive
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If the interlace mode in the video info corresponding to a buffer is
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**"progressive"**, then the buffer is progressive.
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### Plain Interlaced
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If the video info interlace mode is **"interleaved"**, then the buffer is
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plain interlaced.
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`GST_VIDEO_BUFFER_FLAG_TFF` indicates whether the top or bottom field
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is to be displayed first. The timestamp on the buffer corresponds to the
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first field.
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`GST_VIDEO_BUFFER_FLAG_RFF` indicates that the first field (indicated
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by the TFF flag) should be repeated. This is generally only used for
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telecine purposes but as the telecine state was added long after the
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interlaced state was added and defined, this flag remains valid for
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plain interlaced buffers.
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`GST_VIDEO_BUFFER_FLAG_ONEFIELD` means that only the field indicated
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through the TFF flag is to be used. The other field should be ignored.
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### Telecine
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If video info interlace mode is **"mixed"** then the buffers are in some
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form of telecine state.
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The `TFF` and `ONEFIELD` flags have the same semantics as for the plain
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interlaced state.
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`GST_VIDEO_BUFFER_FLAG_RFF` in the telecine state indicates that the
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buffer contains only repeated fields that are present in other buffers
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and are as such unneeded. For example, in a sequence of three telecined
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frames, we might have:
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AtAb AtBb BtBb
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In this situation, we only need the first and third buffers as the
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second buffer contains fields present in the first and third.
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Note that the following state can have its second buffer identified
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using the `ONEFIELD` flag (and `TFF` not set):
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AtAb AtBb BtCb
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The telecine state requires one additional flag to be able to identify
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progressive buffers.
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The presence of the `GST_VIDEO_BUFFER_FLAG_INTERLACED` means that the
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buffer is an 'interlaced' or 'mixed' buffer that contains two fields
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that, when combined with fields from adjacent buffers, allow
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reconstruction of progressive frames. The absence of the flag implies
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the buffer containing two fields is a progressive frame.
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For example in the following sequence, the third buffer would be mixed
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(yes, it is a strange pattern, but it can happen):
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AtAb AtBb BtCb CtDb DtDb
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### Alternate fields
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Since: 1.16
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If the video info interlace mode is **"alternate"**, then each buffer
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carries a single field of interlaced video.
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`GST_VIDEO_BUFFER_FLAG_TOP_FIELD` and `GST_VIDEO_BUFFER_FLAG_BOTTOM_FIELD`
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indicate whether the buffer carries a top or bottom field. The order of
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buffers/fields in the stream and the timestamps on the buffers indicate the
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temporal order of the fields.
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Top and bottom fields are expected to alternate in this mode.
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Caps for this interlace mode must also carry a `format:Interlaced` caps feature
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(`GST_CAPS_FEATURE_FORMAT_INTERLACED`) to ensure backwards compatibility for
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the new mode.
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The frame rate in the caps still signals the frame rate, so the notional field
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rate will be twice the frame rate from the caps (see `GST_VIDEO_INFO_FIELD_RATE_N`).
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